I'll bet a nickel I don't have to tell you which one of these I think might be a satin proof. They're both graded MS67 (PCGS left, NGC right).
From the view of the edge, it appears to be chamfered for a business strike, so I would not feel it is a proof coin. Pretty though. Jim
Both are lacking in the strike for a satin proof, nearly all of which will show 100% full detailing in the area immediately above the braid ribbon and in the bison's fur on the reverse.
Mint State: http://www.pcgscoinfacts.com/Coin/Detail/3977 Satin Proof: http://www.pcgscoinfacts.com/Coin/Detail/3994 Compare the definition in the lettering on the coins. There's the most obvious "tell" on this one. Don't expect big square rims to jump out at you on these.